Ultra-high molecular weight polyethylene (UHMWPE) is the most commonly used bearing material in total joint replacements and was introduced by John Charnley in the early 1960s (The UHMWPE Handbook, edited S. Kurtz, Elsevier, 2004). Since then, a wide variety of applications have been developed in the total joint arthroplasty, as a result of the material's high toughness and good mechanical properties. Although “conventional” UHMWPE has an excellent clinical record, the maximum lifetime of implant systems is restricted due to the wear particles released from the UHMWPE bearing surface (Willert H. G., Bertram H., Buchhorn G. H., Clin Orthop 258, 95, 1990). These wear particles can induce an osteolytic response in the human body leading to local bone resorption and eventually to aseptic loosening of the artificial joint. A second problem associated with conventional, gamma-sterilized UHMWPE, is the oxidative degradation that occurs during shelf ageing. The energy of the gamma rays is sufficient to break some of the carbon-carbon or carbon-hydrogen bonds of the polyethylene chains resulting in the formation of free radicals. These radicals partially recombine but some of them are long-living and can react with oxygen present in, or diffusing into, packaging surrounding the implant (Costa L., Jacobson K., Bracco P., Brach del Prever. E. M., Biomaterials 23, 1613, 2002). The oxidative degradation reactions lead to embrittlement of the material and therewith reduce the mechanical properties of the material and might lead to fracture of the implant (Kurtz S. M., Hozack W., Marcolongo M., Turner J., Rimnac C., Edidin A., J Arthroplasty 18, 68-78, 2003).
In the 1970s, highly crosslinked UHMWPEs have been introduced with the intention of improving the wear resistance of the material (Oonishi H., Kadoya Y., Masuda S., Journal of Biomedical Materials Research, 58, 167, 2001; Grobbelaar C. J., du Plessis T. A., Marais F., The Journal of Bone and Joint Surgery, 60-B, 370, 1978). The UHMWPE materials were gamma irradiated at high doses (up to 100 Mrad, this in contrast to gamma sterilization at ˜2.5 Mrad) to promote the crosslinking process in the material and thereby increase the wear resistance. The free radical amount on the polyethylene chains is not or only locally reduced, however, and therefore these materials are prone to oxidative degradation during shelf ageing or in-vivo use.
More recently, the irradiation crosslinking processes have been extended by a thermal treatment to reduce or eliminate the number of free radicals. These processes can be subdivided into three groups:                Irradiation below the melting temperature followed by annealing below the melting temperature (U.S. Pat. No. 5,414,049, EP0722973). The main disadvantage of this route is the fact that the UHMWPE chains still contain residual free radicals which lead to oxidative degradation (Wannomae K. K., Bhattacharyya S., Freiberg A., Estok D., Harris W. H., Muratoglu Arthroplasty, 21, 1005, 2006).        Irradiation below the melting temperature followed by remelting above the melting temperature (U.S. Pat. No. 6,228,900). The main disadvantage of this processing scheme is that compared with the annealing process, the mechanical properties are reduced by the remelting step (Ries M. D., Pruitt L., Clinical Orthopaedics and Related Research, 440, 149, 2005).        Irradiation in the melt (U.S. Pat. No. 5,879,400, Dijkstra D. J., PhD Thesis, University of Groningen, 1988). The disadvantage of this process is that the crystallinity is substantially reduced and therewith the mechanical performance.        
As a next step, chemical antioxidants have been introduced into medical grade UHMWPE to obtain a wear resistant material that combines a good oxidative stability with sufficient mechanical properties. Most of the common antioxidants exhibit reduced or no biocompatibility, and therefore chemical substances already existing in the human body or in nutritional products were sought. In 1982, Dolezel and Adamirova described a procedure to increase the stability of polyolefins for medical implants against biological degradation in living organisms (CZ 221404). They added alpha-, beta-, gamma- or delta-tocopherol (vitamin E), or a mixture thereof, to polyethylene resin and subsequently processed the resulting mixtures. Besides vitamin E, another class of biologically harmless substances was introduced as oxidation stabilizers in polyethylenes: Hahn described the doping of UHMWPE with carotenoids (e.g. β-carotene) to produce stable and oxidation resistant medical implants (U.S. Pat. No. 5,827,904). However, the wear and oxidation properties of irradiation crosslinked, β-carotene containing products have not been investigated to date.
Recently, several groups established different processing procedures and combined the addition of vitamin E with a radiation crosslinking step to improve the wear resistance of the material (WO 2005/074619). Several investigators added the vitamin E prior to the consolidation of the UHMWPE powder (JP 11239611, U.S. Pat. Nos. 6,277,390, 6,448,315, WO0180778); others diffused the liquid vitamin E into machined products, occasionally with the aid of elevated temperatures (CA 256129, WO 2004064618, WO 2005110276).
Disadvantages of the first of these techniques is the production of a material with lower crosslink density (leading to products with reduced wear resistance) compared with non-stabilized UHMWPE, due to the radical-absorbing properties of the added vitamin E during the actual crosslinking process. Another disadvantage of the processes in WO0180778 is the fact that the implant is machined from a preform that contains vitamin E, this implant is packaged and subsequently irradiated at relatively high doses (>4 Mrad) which will lead to an increased density of the implant and therefore negatively effect the dimensional stability of the implant. Furthermore, the packaging material is exposed to higher irradiation doses which might decrease the long-term mechanical or barrier properties of the packaging. More preferably, the preformed block or rod is irradiated at higher doses and subsequently the implant is machined with high precision from that material and finally packaged. Moreover, the fabrication of homogeneous products with UHMWPE powder and the liquid, highly viscous vitamin E remains challenging.
The second technique also comprises several drawbacks: Due to the diffusion-controlled doping of UHMWPE products, the depth of the vitamin E level remains uncontrolled, inhomogeneous and limited in its spatial dimensions. Although annealing steps after the actual doping process (which is also carried out at elevated temperatures) partially solve the problem of concentration gradients, the final amount of vitamin E in finished products remains unknown.
Stabilization of polyolefins is not restricted to artificial joint applications. Applications could also be in other medical devices like syringes, blood bags, drug vials, medical packagings and the such. But also food contact applications like food packagings, plastic dishes or agricultural and nutritional applications like green houses, liners for food containers and other consumer durables are possible. Even other applications like tubings, fibers, monofilaments or products for the textile industry, but also applications in the building, automotive or electrical industry contain different stabilizers. In these products, polyolefins such as high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and polypropylene are widely used and are stabilized using butylated hydroxytolene (BHT), Irganox 1010, Irganox B 215 or the such. These additives prevent the polymer system from aging due to e.g. UV or visible light, chemical, physical, mechanical or thermal degradation or other environmental influences such as moisture. For these applications, the polyolefins containing the additives are not necessarily subjected to a gamma or e-beam crosslinking step, also non-crosslinked additive containing materials might be used. For other applications it could be useful though to crosslink the polyolefin containing the natural antioxidants. Examples of such applications are tubes demanding an improved thermal stability or heat shrinkable tubes. Of course, many other applications are possible.